This invention relates to a composite powder based on substrate particles coated with micronized metal oxides particles. The composite powder is particularly useful in cosmetics and skin care compositions. Most specifically, the invention relates to compressed powder cosmetics effective in protecting the skin against ultraviolet (UV) rays.
Known compressed powder foundations characterized by anti-ultraviolet protection incorporate in their formulations micronized titanium dioxide having a mean particle size of less than 100 nm. To achieve the desired protection from harmful ultraviolet rays, as much as 10% by weight or more of micronized titanium dioxide is used. The use of high amounts titanium dioxide having a mean particle size of less than 100 nm tends to result n agglomeration in the cosmetic, which in turn diminishes the protection against UV rays write also making the composition excessively opaque on the skin, a phenomenon known as "whitening effect". In addition, such compressed powder foundations have a scratchy and rough feel during and after application to the skin.
Another approach for imparting anti-UV protection to cosmetics is by incorporating liquid organic UV screeners such as octyl dimethyl PABA and ethylhexyl p-methoxy cinnamate. However, the use of such liquid UV screeners is limited to oil-based lotions and emulsion-type cosmetics. It is not possible to attain the desired protection from UV rays by using liquid organic UV screeners alone in compressed powder foundations.
Compressed powder foundations containing a composite of micronized titanium dioxide and thermoplastic bead-shaped powders are also known. A composite material is generally known as a complex material in which two or more individual substances are combined by physical and/or chemical means to form a single homogeneous material possessing structural and/or functional properties not present in any of the individual components. In the abovementioned known composite of titanium dioxide and thermoplastic bead-shaped powders, as much as 30% by weight of the micronized titanium dioxide powder can be embedded on the surface of the thermoplastic powders. To attain the desired protection, a compressed powder foundation must contain up to 20% of this composite. The use of such relatively large amount of bead-shaped powder in compressed powder foundations not only presents problems with the compressibility of the composition, but also causes instability of the cosmetic in the container during transportation, particularly transportation by the cosmetic user. In addition, since the shape of the beads is spherical, the bead-shaped powders come off from the skin very easily and do not provide prolonged protection from UV rays when such protection is essential.
A method has been known for preparing and using titanium coated mica in published unexamined Japanese Patent Application (Application No. 5-87545). This method discloses mica only as a substrate and focuses on the relative transparency of the coated pigment as compared to conventional titanium coated mica or pearlescent pigment in general. In addition, this method uses titanyl sulfate as a starting chemical to form titanium dioxide, which means all the reaction must be carried out in an aqueous medium. This slurry process involves additional filtration, drying, and calcination at a temperature up to 900.degree. C. to convert the hydrous oxide coating to crystalline oxide. Because of the high temperature of the calcination involved in this process, the base substrate is limited to materials capable of being stable at such high temperature. Moreover, the cost associated with the series of steps involved makes the use of the micronized titanium dioxide coated mica too expensive as a cosmetic substrate.
A method for preparing and using predispersed pigments on the flaky particulate by using a so-called "ordered mixture" mechanism is also known and is disclosed in a laid-open Japanese Patent Application (Application No. 5-214257). This method only concerns the dispersion mechanism of pigments having a particle size of less than 5 micron in substrates having an aspect ratio of 10 to 120. There is no indication in this method of the use of micronized titanium dioxide as a coating agent serving as a UV screener, and this method does not describe the use of a binder system.
U.S. Pat. No. 4,772,331 discloses a method for preparing colored flaky pigments comprising flaky substrates having a finely divided color pigment material adhered on the surfaces thereof by means of a high molecular weight organic binder. The patent limits the binder to high molecular weight organic compounds such a polyethylene glycols, polypropylene glycols or polyvinyl pyrrolidones having a molecular weight of from about 500 to 160,000. Since all the reaction must take place in the liquid medium, this method presents not only the potential problem of agglomeration of the colored pigments during the drying process, but also the problem that the cost associated with the filtration, drying, and pulverizing steps makes the material produced from this process too expensive for use as a cosmetic substrate.